Serialise Google Analytics Custom Metric?

I am trying to set a custom metric counting the number of submissions of a form. I would like this metric to count only once per session. However, Google Analytics only offers scop "Hit" or "Product" for custom metrics. None of these will do the job.

(Sessions in GA are actually "Entrances" for any Hit-based Dimensions!)

So if you want to track a form completion into a Custom Metric, and the name of the Form into a Session-scoped Custom Dimension (because you want to count each form just once per visit), what happens if the user fills in two forms?

The Session-scoped Custom Dimension for the Form Name will only report the last form that was filled in, however the Custom Metric “Form Completions” will count 2 Completions. So it looks as if that second form was filled in twice when in fact the second completion stemmed from an entirely different form....